The present disclosure relates generally to drilling systems and more particularly to downhole tools.
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present techniques, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Wells are generally drilled into the ground or ocean bed to recover natural deposits of oil and gas, as well as other desirable materials that are trapped in geological formations in the Earth's crust. A well is typically drilled using a drill bit attached to the lower end of a “drill string.” Drilling fluid, or “mud,” is typically pumped down through the drill string to the drill bit. The drilling fluid lubricates and cools the drill bit, and it carries drill cuttings back to the surface in an annulus between the drill string and the borehole wall.
For successful oil and gas exploration, it is beneficial to have information about the subsurface formations that are penetrated by a borehole. For example, one aspect of standard formation evaluation relates to measurements of the formation pressure and formation permeability. These measurements are valuable for predicting the production capacity and production lifetime of a subsurface formation.
One technique for measuring formation properties includes lowering a “wireline” tool into the well to measure formation properties. A wireline tool is a measurement tool that is suspended from a wire as it is lowered into a well so that it can measure formation properties at desired depths. A typical wireline tool may include a probe or packer inlet that may be pressed against the borehole wall to establish fluid communication with the formation. This type of wireline tool is often called a “formation tester.” A formation tester measures the pressure of the formation fluids and generates a pressure pulse, which is used to determine the formation permeability. The formation tester tool also typically withdraws a sample of the formation fluid for later analysis.
In order to use a wireline tool, whether the tool is a resistivity, sampling, porosity, or formation testing tool, the drill string is removed from the well so that the tool can be lowered into the well. This is called a “trip” downhole. Further, wireline tools must be lowered to the zone of interest, generally at or near the bottom of the hole. A combination of removing the drill string and lowering the wireline tools downhole are time-consuming measures and can take up to several hours, depending upon the depth of the borehole. Because of the expense and rig time involved to “trip” the drill pipe and lower the wireline tools down the borehole, wireline tools are generally used only when the information is greatly desired, or when the drill string is tripped for another reason, such as changing the drill bit.
As an improvement to wireline technology, techniques for measuring formation properties using tools and devices that are positioned near the drill bit in a drilling system have been developed. Thus, formation measurements are made during the drilling process, and the terminology generally used in the art is “MWD” (measurement-while-drilling) and “LWD” (logging-while-drilling). MWD typically refers to measuring the drill bit trajectory, as well as borehole temperature and pressure, while LWD typically refers to measuring formation parameters or properties, such as resistivity, porosity, permeability, and sonic velocity, among others. Real-time data, such as the formation pressure, allows the drilling entity to make decisions about drilling mud weight and composition, as well as decisions about drilling rate and weight-on-bit, during the drilling process.
Downhole tools can be complex and inefficient. For example, multiple moving parts involved in a formation testing tool, such as MWD and LWD tools, can result in less than optimal performance. Further, at significant depths, substantial hydrostatic pressure and high temperatures are experienced, thereby further complicating matters. Still further, formation testing tools are operated under a wide variety of conditions and parameters that are related to both the formation and the drilling conditions. Therefore, there is a need for improved downhole formation evaluation tools and improved techniques for operating and controlling downhole formation evaluation tools so that these tools are more reliable, efficient, and adaptable to formation and mud circulation conditions.